The Medicare enrollment process is fraught with complicated choices -- but one big decision could be made for you automatically by a health insurance company if you are not on your toes.

A little known rule allows some health insurance companies to automatically sign people up for Medicare Advantage plans -- the managed care alternative to traditional Medicare. Under so-called seamless conversion, an insurer can seek permission
from the Centers for Medicare & Medicaid (CMS) to auto-enroll members of its non-Medicare insurance plans when they reach 65, the age of Medicare eligibility.

Auto-enrollment might be convenient, but it short circuits one of the most important choices to be made when signing up for Medicare -- whether to go with traditional fee-for-service Medicare, or an Advantage plan.

Advantage plans offer some cost saving and usually include prescription drug coverage. But the majority of Medicare enrollees use traditional Medicare. And thorough shopping for an Advantage plan requires a careful review of the networks of health providers and prescription drug coverage to make sure they are the best fit for an individual's health needs.

Congress gave the green light for seamless conversion enrollment in 1997, and CMS established rules to allow them in 2005. Health insurance companies can seek permission to carry out seamless conversions of members of their individual, public exchange or Medicaid managed care plans.

Medicare has not released information on the prevalence of seamless conversion. But the practice is drawing scrutiny from consumer advocate groups, who are pushing for more transparency from CMS on the companies and locations where the process has been approved.

"It flew under the radar for quite a long time, and it's just beginning to gain more attention," said Gretchen Jacobson, associate director of the program on Medicare Policy at the Kaiser Family Foundation.

A Medicare spokesman said data will be released later this year indicating which insurance companies are doing seamless conversion, and where. CMS also is "refining" its procedures for processing seamless conversion applications from insurance companies.

Some advocates also are urging CMS to strengthen the rules on notifications that must be sent to enrollees who find themselves automatically signed up in an Advantage plan. The current rules require insurance companies only to provide a notification of enrollment by mail 60 days prior to seamless conversion. The notification must include clear information on how to opt out before coverage begins. But mailboxes typically overflow with Medicare marketing pitches around one's 65th birthday, making it easy to miss a notice of seamless

"Our preference would be that the process requires an active choice, that you must respond to an invitation for a seamless enrollment," said Joe Baker, president of the Medicare Rights Center, a consumer advocacy organization.